This invention relates to a sonar towed array and, more particularly, to a ribbon termination member for a high-strength, thin-line towed sonar array.
Generally, sonar arrays are towed up to a mile or so behind the towing vessel. The towed array is deployed by unwinding it from the spool of a winch, on which it is stowed when not in use. The array may be on the order of 1500 feet in length and one inch in diameter, and is generally made in sections or modules of about 90 feet in length, which are joined together by means of an electro-mechanical coupling. An internal strength member runs the length of each module, and functions to carry the tensile load on each module, mainly caused by drag of the array as it is towed.
Some modules are acoustic modules, and contain sensitive acoustic sensors. In order to provide space for the sensors and to prevent unwanted acoustic signals from being passed to these sensors, a flat or ribbon-type strength member is employed in the acoustic modules. This ribbon-type strength member is arranged inside the walls of the tubular jacket of the module so that it does not have mechanical contact with the sensors, to minimize the transfer of unwanted acoustic signals.
Other modules, called vibration isolation modules, function to mechanically isolate the acoustic-type modules from vibrations of the towing vessel. This results in reducing the transfer of unwanted acoustic signals to the acoustic sensors contained in the acoustic modules.
The vibration isolation module contains no acoustic sensors and therefore can use a more economical braided rope type or round strength member, rather than the lower-profile flat, ribbon-type strength member which provides space for the sensors.
The maximum length of an electro-mechanical coupling, and therefore the maximum length of a strength termination member, is limited in order to prevent damage to the array which may occur during winching by wrapping the rigid straight coupling around the curved winch drum. The maximum diameter of a strength termination member is also limited because it must fit inside the electro-mechanical coupling, which itself must carry the entire load of the array. In the preferred embodiment and in present practice, the array is limited to a maximum diameter of 1 inch.
The strength termination member must provide space for containing a multi-pin potted electrical connector and allow for passage of coaxial cables.
The strength termination member must also ensure the integrity of the strength member during and after repeated deployment and retrieval winching operations.
Kevlar aramid fiber material, a synthetic material produced by E. I. DuPont de Nemours & Co., is frequently used as the strength member in military and commercial towed sonar and sounding arrays. See U.S. Pat. No. 4,160,229 to McGough, and U.S. Pat. No. 4,090,168 to Miller, et al. Kevlar fiber material is an aramid, which is the generic name for a distinctive class of aromatic polyamide fibers. Kevlar aramid fiber material has an extremely high tensile strength, and greater resistance to elongation than steel. Its high energy-absorption property makes it particularly suitable for use as belting in radial tires and as a reinforcing material for polymer composites.
Although Kevlar aramid fiber material is very strong in tension, it is subject to damage by being compressed, kinked, cut or bent around a tight radius. Such damage weakens the Kevlar aramid fiber material, leading to early failure when used as a strength member in a towed sonar array. This is particularly the case when the "wedge-cone" type of termination is employed on a Kevlar aramid fiber material strength member in a towed array. In the "wedge-cone" termination, the ends of several Kevlar aramid fiber material ribbons are inserted in a conical opening in a metal termination, and a metal cone is driven into the opening to wedge the Kevlar aramid fiber material therein, by pinching the ribbons against the side of the opening.
Uneven lengths of the Kevlar aramid fiber material strength member ribbons, common with this type of termination, cause one of the ribbons to take a disproportionately greater percentage of the load, resulting in early failure. Also, during winching operations where the pull is not straight, the Kevlar aramid fiber material ribbons may be bent and pulled taut over a sharp edge or a tight radius, causing damage to the Kevlar strength member.